1. Field of the Invention
This invention relates to radio frequency (RF) ion guides for use in mass spectrometry.
2. Description of the Related Art
Ion guides are commonly used in a mass spectrometer (MS) to transport ions between the ion source and the mass analyzer and commonly consist of a number of elongate, parallel conductive rods that are placed around a common axis. Various embodiments of ion guides are known in the art. An example of a prior art multipole ion guide is illustrated in FIG. 1. For convenience of description, the ion guide example of FIG. 1 is specific to a quadrupole ion guide. However, embodiments of the invention may be used also in other types of multipoles, such as hexapoles, octopoles, etc. In the ion guide of FIG. 1, ions from an ion source not shown in the figure are transferred to the ion guide 100, which is driven by voltage generators 105 and 110.
As shown in FIG. 1, four conductive rods, constituting the quadrupole ion guide 100, are arranged in two pairs, each pair receiving the same RF signal denoted as V cos(wt), wherein V and w are the magnitude and frequency of the RF signal, respectively. One pair of rods receives the signal at zero phase (+V cos(wt)) while the other receives the signal at a 180 degrees phase shift (−V cos(wt)), whereby the ion guide 100 acts as an ion tube transmitting the ions over a broad range of mass to charge ratios, generally denoted as m/z. The range of m/z ratios has a lower and an upper limit beyond which ions cannot be reliably transmitted anymore. While the lower limit is quite sharp (sometimes called lower mass cut-off), the upper limit is slightly more diffuse.
FIG. 2 schematically shows an example of a quadrupole ion guide Q0 for transporting the ions prior to a triple quadrupole mass analyzer assembly Q1, Q2, Q3 in the wider context of a complete mass spectrometer. The mass spectrometer may be mounted in a housing 200, which is divided in two separate vacuum stages 202A, 202B, and may comprise an El or Cl ion source 204, a lens tube 206 at the exit of the ion source 204 for extracting ions and transmitting them to the quadrupole ion guide Q0, a primary mass filter Q1, a curved quadrupole collision/fragmentation cell Q2 providing a U-turn of the ion path, and a secondary mass filter Q3 in serial alignment between the ion source 204 and an ion detector.
As shown, ion source 204 and ion detector may generally be provided at opposing ends of the ion path in the mass spectrometer. Due to the particular path settings in the example shown, the ion source 204 and the ion detector can be located immediately adjacent to one another, separated only by intermediate walls 208 (dashed lines) bordering the two vacuum stages 202A, 202B. Deviating from the example shown, it would be likewise possible to replace the curved assemblies Q0 and Q2 by straight equivalents, whereby a linear configuration would result.
An ultra-high (turbo) vacuum pump, not shown, may be disposed in the housing 200 to maintain the two vacuum stages 202A, 202B evacuated. Evacuation holes, not shown in FIG. 2, may be provided at different positions of the housing 200. Lens tube 206 and ion source 204 are positioned in a first sealed region of the housing 200 provided by the walls 208 and a sealing ring which engages a cover, both not shown, to provide the vacuum seal.
At the center of the ion path along the quadrupole ion guide Q0, a gas inlet may be provided for introducing an interaction gas, such as helium, nitrogen or methane, into the quadrupole ion guide Q0 which can be configured, for example, like the one described in U.S. Pat. No. 8,525,106 B2 to Muntean.
In the example shown in FIG. 2, the quadrupole ion guide Q0 is curved by 90°. Radio frequency voltages and, as the case maybe, direct current (DC) offset voltages, can be applied to adjacent pole electrodes. The pole electrode profile may be of several different shapes, such as square, circular round, hyperbolically round, circular concave, flat, rectilinear, etc.
Because of the proximity to the source region, ion guides are generally exposed to contamination in the form of deposits on the ion guide electrodes. Deposits can be brought about by either neutral molecules that condense on the electrodes, or by large amounts of ions that are rejected by the ion guide, hit the electrodes as a result thereof and lose the charge so that the underlying neutral substrate molecules condense on the electrodes. A combined effect of the aforementioned may also be that neutral molecules condense on the electrodes and then react with rejected ions that hit the electrodes and decompose into stable solid structures that “grow” on the electrode surfaces, such as carbon deposits originating from hydrocarbon analyte molecules that have decomposed.
FIG. 3 illustrates by way of example a graphical representation of deposits (dashed contours) that have actually formed on the inner surfaces of two quadrupole ion guide electrodes during operation in the laboratory of the inventors. Shown is an entrance end, where ions are received along the central trajectory arrow, of the two pole electrodes that have a substantially square cross section. For the sake of clarity, the other two quadrupole electrodes which would normally be positioned opposite the depicted ones in order to enable radial ion confinement are not shown. The two dashed arrows diverging from the central dashed arrow schematically indicate the path that rejected ions would have taken, in contrast to ions that would have been transmitted. As is evident from this illustration and frequently observed in laboratory practice, the deposits form predominantly at the central parts of the electrode surface.
Such deposits in mass spectrometers are described in the literature, for example, by Girard et al. (Journal of Chromatography Science, 2010 October, 48 (9), 778-779) and Kenneth L. Busch (“Ion Burn and the Dirt of Mass Spectrometry”, online publication, Sep. 1, 2010).
The formation of deposits on the ion guide electrodes is undesired, because the deposited layer may be dielectric and charges up when hit by rejected ions. In such a case, the deposits can lead to undesired electric potential barriers which deflect and distort ion motion and deteriorate the MS performance.
A remedy for the above-mentioned deposition problem could be to heat the ion guide electrodes during operation so that they are less prone to accepting contaminating deposits. Another remedy would be to periodically clean the ion guide electrodes in order to restore the MS performance when the deposits have grown too large. The first solution, heating, adds complexity to the mass spectrometer design, both because it requires additional hardware for heating and because it requires adding a heat barrier to prevent the hot ion guide from affecting the performance of the mass analyzer that follows. The second solution, cleaning, is generally not desired at high frequency, because it reduces the uptime of the instrument and is thereby detrimental to the productivity of the MS. Furthermore, it may also create performance problems if disassembly, cleaning, and reassembly are not carried out correctly (for instance by ill-trained staff).
D. L. Swingler, International Journal of Mass Spectrometry and Ion Processes, 54 (1983) 225-230, suggested to provide for longitudinal or transverse slots in the pole electrodes of a quadrupole mass filter. Although such structural modification of the electrodes might mitigate the contamination issue, the electrodes retain material directly at their front ends which are particularly susceptible to ion impingement and hence deposit forming. It can be shown from ion trajectory simulations that the electrode surfaces at the entrance area of an ion guide are exposed to the highest ion current, because most ions rejected by the RF confinement fields (that is, not stably transmitted) will be ejected at this point.
In view of the foregoing, there is a need to provide ion guides that are less susceptible to contamination on the electrode surfaces.